poisonous fangs often have some characteristic either of rattle, hood, or unusual colour, which indicates that they had better be left alone.
But there is yet another form of coloration, which consists in special markings—bands, spots, or patches of white, or of bright colour, which vary in every species, and are often concealed when the creature is at rest but displayed when in motion,—as in the case of the bands and spots so frequent on the wings and tails of birds. Now these specific markings are believed, with good reason, to serve the purpose of enabling each species to be quickly recognised, even at a distance, by its fellows, especially the parents by their young and the two sexes by each other; and this recognition must often be an important factor in securing the safety of individuals, and therefore the wellbeing and continuance of the species. These interesting peculiarities will be more fully described in a future chapter, but they are briefly referred to here in order to show that the most common of all the characters by which species are distinguished from each other—their colours and markings—can be shown to be adaptive or utilitarian in their nature.
But besides colour there are almost always some structural characters which distinguish species from species, and, as regards many of these also, an adaptive character can be often discerned. In birds, for instance, we have differences in the size or shape of the bill or the feet, in the length of the wing or the tail, and in the proportions of the several feathers of which these organs are composed. All these differences in the organs on which the very existence of birds depends, which determine the character of flight, facility for running or climbing, for inhabiting chiefly the ground or trees, and the kind of food that can be most easily obtained for themselves and their offspring, must surely be in the highest degree utilitarian; although in each individual case we, in our ignorance of the minutiæ of their life-history, may be quite unable to see the use. In mammalia specific differences other than colour usually consist in the length or shape of the ears and tail, in the proportions of the limbs, or in the length and quality of the hair on different parts of the body. As regards the ears and tail, one of the objections by Professor